RMR vs BMR for Weight Loss

RMR and BMR, the former abbreviated from Resting Metabolic Rate and the latter from Basal Metabolic Rate, can often be encountered in discussions related to daily caloric requirements, dieting, caloric restrictions and weight loss. Fitness amateurs and professional athletes and their physicians might also use RMR, BMR, or both for weight management.

This article will cover the differences between RMR and BMR and discuss their practical application in weight management with a focus on weight loss.

RMR vs BMR – a Side-by-Side Comparison

BMR and RMR both measure the energy expended by your body while at complete rest, and in both cases measurements are taken using indirect calorimetry hence the result is in calories expended per day. However, the conditions for taking BMR measurements are slightly more strict compared to those of RMR measurements, leading to RMR accounting for a bit more physical activity than BMR. As a result, your RMR is typically slightly larger than your BMR.

A side-by-side comparison of the similarities and differences between RMR and BMR are presented in the table:

Characteristic	BMR	RMR
What it measures	The bare minimum energy needed to keep you alive and awake. It does not include any activity other than lying flat, not even digestion.	The energy needed to keep you alive and at rest, though depending on measurement it may include some after effects of physical activity and food induced thermogenesis.
Measurement unit	Typically Calories or kilocalories per day. Sometimes calculated in kcal·m-2·h-1 – kilocalories per body surface area per day.	Typically Calories or kilocalories per day. Sometimes calculated in kcal·m-2·h-1 – kilocalories per body surface area per day.
Measurement conditions	1) after 10-12 hour fast 2) no intense physical activity 12h before measurement 3) lying down fully awake 4) thermal-neutral environment 5) stress-free, familiar with equipment	1) after 4-5 hour fast 2) no physical activity at least 20 minutes before measurement 3) lying down fully awake 4) thermal-neutral environment
Use in weight loss	To set a baseline for dietary restrictions. Going below BMR for any length of time equals starvation.	To set a baseline for dietary restrictions. Going below RMR for any length of time equals starvation.

The RMR measurement conditions are based on a study funded by the American Dietetic Association ^[1] and BMR measurement conditions are based on a paper on Basal metabolic rate studies ^[2].

Even the small differences above might disappear in certain cases. For example, in Mindy L.L. et al.’s study on RMR measurement reliability ^[3] they instructed subjects to fast overnight. They also erased the first minutes of the measurement to account for stress induced by the unfamiliar environment and measurement machinery. This means that the RMR obtained was very close to the basal metabolic rate.

Can RMR be used instead of BMR?

Even though the period of fasting is longer for BMR compared to RMR, the thermic effect of food is almost negligible after the first couple of hours ^[1]. Likewise, unless one had undergone intense exercise, the longer-term effects of that would not be very pronounced ^[1].

Given that the measurement method for both RMR and BMR is the same (typically indirect calorimetry), and the conditions mostly the same, it is not surprising to discover that they differ very little in practice, typically well within the measurement error of both.

Even more importantly, in practice most would use RMR and BMR equations to estimate their energy expenditure in Calories (also why RMR is often called REE – Resting Energy Expenditure), meaning that the above differences apply even less.

If you are using an RMR calculator and compare the results from that of a BMR calculator, you will find that the results are almost identical, confirming the conclusion that in practice, RMR and BMR can be used interchangeably, including in TDEE calculations and in estimating your daily energy requirements, daily calorie needs, and weight management.

How Accurate and Reliable are RMR & BMR Measurements?

While the accuracy is difficult to assess externally, as RMR and BMR are the best measurement we have, reliability can be assessed through repeated measurements. Compher et al. ^[1] answer the question “What differences in RMR are seen when measuring the same individual at various times of the day or on different days?” with:

Repeated measures vary 3% to 5% over 24 hours (Grade II) and up to 10% over weeks to months.

So you can expect a laboratory measurement of RMR to vary between 3 to 5% of a middle value, and you may expect it to change slightly over time.

Using RMR and BMR in Weight Loss

Weight loss is typically about the mathematics of energy intake and energy expenditure. Spend more energy than you ingest through food and beverages, and a loss of weight is an inevitable consequence. Therefore, a lot of weight loss literature focuses on restricting the calorie intake of a person, or so-called calorie counting.

Some coaches, nutritionists, and physicians use RMR and BMR estimates in order to get a rough guidance as to the bare minimum caloric intake of a person, typically based on easily obtainable measures such as weight and height, as well as sex and age. Then, the total daily energy expenditure (TDEE) is estimated by applying a multiplier to either RMR or BMR to arrive at a daily caloric requirement to maintain weight. See TDEE vs BMR for an in-depth comparison.

Then, typically a fixed amount of calories is subtracted from said TDEE in order to set a limit on the number of calories ingested per day, leading to weight loss, if the regiment is adhered to strictly. For example, a reduction of 500 Calories per day or 3500 Calories per week is assumed to be needed to achieve a loss of one pound a week.

However, nowadays there are more advanced methods for setting the daily caloric limit to achieve loss of weight that take many more factors into account. They put the 3500 Calorie per week myth to rest, and may lead to better weight management outcomes. If weight loss is your primary goal of researching this topic, you’d want to try our free online weight loss calorie calculator.

Importantly, before you make any changes to your diet, energy intake, or sources of energy, you should consult a physician, nutritionist, or another specialist to avoid endangering your health.

Can You Increase Your RMR or BMR Through Regular Exercise?

While you might expect your RMR and BMR, even measured under highly controlled conditions, to change slightly over time, there is no conclusive evidence that you can influence such a change by physical training or exercise ^[4]:

Many studies have shown that long-term training increases RMR, but many other studies have failed to find such effects. Data concerning long-term effects of training are potentially confounded by some studies not leaving sufficient time after the last exercise bout for the termination of the long-term EPOC.

Given that the average daily energy expenditure is in the 1,500 – 2,000 Calorie range, a difference of 5% is around 100 Calories. Likewise, the difference between a person’s RMR and their BMR are likely to be in the 100-200 Calorie range. Therefore, for weight loss purposes, using either RMR or BMR should work well. After all, it is unreasonable to expect someone to be able to follow a daily diet as strictly, except for maybe top professional athletes.

References:

[1] Compher C, Frankenfield D, Keim N, Roth-Yousey L; Evidence Analysis Working Group (2006) “Best practice methods to apply to measurement of resting metabolic rate in adults: a systematic review.” Journal of the American Dietetic Association. 106(6):881-903; DOI: 10.1016/j.jada.2006.02.009
[2] Henry CJK, (2005) “Basal metabolic rate studies in humans: Measurement and development of new equations.”, Public health nutrition. 8:1133-52.
[3] Mindy L.L. et al. (1987) “Resting Metabolic Rate: Measurement Reliability”, Journal of Parenteral and Enteral Nutrition, 11(4):354-9; DOI: 10.1177/0148607187011004354
[4] Speakman J.R., Selman C. (2003) “Physical Activity and Resting Metabolic Eate”, The Proceedings of the Nutrition Society, 62(3):621-34; DOI: 10.1079/PNS2003282